This invention relates to digital communication systems, but more specifically to a class of coders used in such systems known as differential coders employing prediction coding techniques for increasing the effective rate of transfer of digital data and/or improving the signal-to-noise ratio in the communication system.
In a communication system, it is often desirable to compress as much as possible the information content of the transmitted signal without undue degradation of the signal-to-noise ratio. Too much degradation reduces the chance of correctly interpreting, e.g. decoding, the received signal and too much redundancy does not efficiently utilize channel capacity. In the case where the transmitted data is speech information, speech rate reduction generally is accomplished by one of two approaches. One approach is "speech coding" which involves removing redundancy from the speech signal. The second approach is "digital speech interpolation" (DSI) which involves detecting and utilizing redundant or silent intervals during the speech conversation to transmit useful information. This invention is related to the earlier.
Differential coding is an efficient technique for encoding speech signals. In differential coding, a coder encodes and transmits, in digital form, the speech data over a data channel at a much lower bit rate than would otherwise be required if the entire speech data were transmitted. Thus with a given channel capacity, the speech data may be compressed for higher efficiency. The data channel may be constituted by electrical conductors or electromagnetic propagation media which contribute noise in the channel. The quality of the data signal at the receiver is measured by its signal-to-noise (S/N) ratio.
Generally, differential coders reduce successive speech data samples to a corresponding prediction error sample, such as the difference between two successive n-bit speech data samples, and then transmits only a prediction error sample for respective successive samples in the data stream. At the decoder, the prediction error samples are processed in a manner to reconstruct the full speech data samples which then have audible significance. In essense, only the difference between successive speech samples is transmitted and the redundancy is substantially reduced, but not eliminated entirely. Accordingly, there still is room for improvement.
Differential pulse code modulators (DPCM), adaptive differential pulse code modulators (ADPCM), and delta modulators (DM) typify various classes of differential coders. A delta modulator is a special type of DPCM. A background description of such coders can be found in articles entitled "Waveform Quantization and Coding", IEEE Press Book (1976) by N. S. Jayant; "Adaptive Prediction in Speech Differential Encoding Systems" by J. D. Gibson, Proceedings of the IEEE, Vol. 68, No. 4, (April 1980); and "Speech Coding" by J. L. Flanagan, et al IEEE Transactions on Communications, Vol. Com-27, No. 4 (April 1979).
Another technique for compressing the data content of a speech signal is known as "sample robbing" where a selected number of the data signals is periodically removed from the transmitted data stream. Such technique has been proposed by S. J. Campanella in his publication "Digital Interpolation Techniques", Proceedings of the 1978 IEEE National Telecommunications Conference, pp. 14.1.1-14.1.5, December 1978. Obviously, sample robbing somewhat degrades the S/N ration, but even moderate degradation may occasionally be tolerable during heavy traffic loading of the channel where the speech signal, when reconstructed at the receiving end of the channel, is still intelligible in an audible sense. The number of samples which may be robbed, however, is limited by aliasing errors introduced by the channel.
Another technique similar to sample robbing is known as "subsampling", such as that described by N. S. Jayant in the article "Subsampling of a DPCM Speech Channel to Provide Two Self-Contained Half Rate Channels", Bell Systems Technical Journal, 60[4], pp. 501-509 (April 1981). Subsampling, though, does normally take advantage of synchronizing the receiver and transmitter circuits and is subject to the same aliasing limitation mentioned above.
In a number of cases the requirement in speech transmission is intelligibility, and not a very high quality. For such requirements, the low bit-rate coders such as 16 to 24 kb/s DPCM (or ADPCM) coders are quite suitable. At these bit rates sample robbing can be used to reduce the bit rate requirement of the data transmission channel. For example, the performance of a 2-bit-word ADPCM (16 kb/s) with no sample robbing and a 3-bit-word ADPCM coder (24 kb/s) with 50% sample robbing is the same. Hence, if sample robbing is used, the effective bit rate is only 12 kb/s. This is equivalent to a 25% savings.
Further, the same performance could be achieved by using a 3-bit-word DPCM coder with 50% sample robbing. The DPCM coder is much simpler than the ADPCM coder.
Telecommunications (i.e., speech/data) traffic of a typical telephone network varies significantly during various times of the day. During peak hours, a number of calls are blocked due to nonavailability of lines. Sample robbing can be used to accommodate the additional traffic in which case a computer monitors the traffic conditions and decides what percentage of samples should be robbed from the channel to accommodate the additional traffic. When samples are robbed, there is some degradation on the speech quality. This degradation is proportional to the percentage of samples robbed.
Although differential coding and sample robbing each enables reduction of the required bit rate of transmission for a given speech signal, it is envisioned that further improvements in communication systems yet can be made in telecommunication systems employing DPCM and ADPCM coders since there remains some residual redundancy in the signal. To attain this improvement, sample robbing is additionally proposed. Sample robbing in a PCM system is straight forward, but in an DPCM or ADPCM where feedback is involved, complex problems such as prediction of the next sample, synchronization of the transmitter and receiver circuits, and replacement of the robbed sample are encountered. For this reason, present communication systems employing differential coders are not know to employ sample robbing.